Aptitude - Boats and Streams - Discussion

Discussion Forum : Boats and Streams - General Questions (Q.No. 5)
5.
In one hour, a boat goes 11 km/hr along the stream and 5 km/hr against the stream. The speed of the boat in still water (in km/hr) is:
3 km/hr
5 km/hr
8 km/hr
9 km/hr
Explanation:

 Speed in still water = 1 (11 + 5) kmph = 8 kmph. 2

Video Explanation: https://youtu.be/KQX_mA3tcVA

Discussion:
38 comments Page 1 of 4.

A.K.BALAKRISHNAN said:   5 years ago
@All.

Follow this method for all boat question;

Downstream means boat towards the down in this situation we have to find the speed of boat and speed of stream.

Let x be boat speed.
Let y be stream speed.

1) Down stream speed = D/T(downstream time) = x+y

note: distance will be the same as both the streams.

2) Upper stream speed = D/T (upper stream time) = x-y.

Thank you.

Sara said:   8 years ago
x = speed of stream still in water.
y = speed of stream.

downstream = Distance/ X+Y = Time,
= 11/ X+Y = 1hour.

Upstream= Distance/ X-Y = Time.
= 5/X-Y = 1hour.

Cross multiply

11 = X + Y -----(1)eq
5 = X - Y ------(2)eq

We need x, so cancel y.
2X = 16,
X = 8kmph.

Musoyiti said:   4 years ago
My way of solving is that;

Let the speed of the current be x.

So the speed in still water will be 11-x and 5 +x. Now as these both expressions are same so we use linear equation and find the value of x to be 3. So the speed of the current is 3 km/hr. So the speed of the boat in still water is 11-3=8.

Hence it is 8 km/hr.
(2)

Nurun Nobi Khokon said:   6 years ago
Let, downstream (with stream) speed= x.
upstream (against) speed is= y.
Now,
x = speed of the boat+speed of the water.
y = speed of the boat-speed of water.
-----------------------------------------------------------
x+y = 2*speed of the boat.

So, speed of the boat = (x+y)/2 [downstream speed+upstream speed].
(1)

Subaram said:   10 years ago
Here direction along the stream called downstream and against the stream called upstream. In this question they gave along the stream 11 kmhr against the stream 5 kmhr. So speed still in water = 1/2 (a+b) , here a downstream be up stream as for formula.
(1)

Upstream relative speed is u + v=11km/hr

Downstream speed is u-v = 5

Where u = speed of boat in still water and v is speed of stream

Then adding two equations u+v + u-v =11+5
2u=16

Finally, u=8.

Dhanmoni said:   2 years ago
In one hour, the boat goes the downstream = 11 km/h
On the other hand, the boat goes the upstream = 5 km/h,
So, the speed of the b.in still water is = d+uÃ· 2.
= 11+5Ã·2
= 8 km/ h => answer.
(2)

Audry said:   8 years ago
Let U= speed of boat in still water and v=speed of stream then;

Upstream speed, U - V = 5km/hr,
Downstream speed, U + V = 11km/hr,

Eqt,
U + V + U - V = 11 + 5
2U = 16
U = 16/2 = 8km/hr.

Saloni nautiyal said:   1 decade ago
Formula for speed of boat in still water=[(X+Y)+(X-Y)]/2
where X=SPEED OF BOAT IN STILL WATER
Y=SPEED OF STREAM/CURRENT
THEREFORE, [(11+5)+(11-5)]/2=11